1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive employing a voice coil motor comprising a voice coil wrapped around a rotary voice coil yoke comprising a low reluctance end and a high reluctance end.
2. Description of the Prior Art
Referring to FIG. 1, a conventional disk drive typically comprises a disk 2 for storing data in a plurality of radially spaced, concentric tracks 4. A head 6 is actuated radially over the disk 2 in order to write data to or read data from a target track 4. The head 6 is typically attached to a suspension 8 which biases the head 6 toward the surface of the disk 2. The suspension 8 is attached to the distal end of an actuator arm 10 which is rotated about a pivot 12 by a voice coil motor 14. The disk 2 typically has recorded thereon embedded servo wedges 16 which store coarse and fine head position information for positioning the head 6 over a centerline of a target track 4.
As shown in FIG. 2A, a conventional voice coil motor 14 typically comprises a voice coil 18 in the shape of a trapezoid comprising a first side 20A opposite a second side 20B. A current is passed through the voice coil 18 to induce a first magnetic flux 22A and a second magnetic flux 22B along the length of each side (20A and 20B). Because the voice coil 18 is wound up one leg and down the other, the direction of the magnetic flux 22A induced along the first side 20A is opposite the direction of the magnetic flux 22B induced along the second side 20B. A first magnet 24A and a second magnet 24B induce respective magnetic fluxes into and out of the page which are orthogonal to the magnetic fluxes (22A and 22B) induced by the voice coil 18. The orthogonal magnetic fluxes induce a horizontal force on the voice coil 18, thereby rotating the actuator arm 10 about the pivot 12 to move the head 6 radially over the disk 2. The actuator arm""s direction of rotation (clockwise or counter-clockwise) depends on the direction of the current passing through the voice coil 18 (clockwise or counter-clockwise). Thus, the direction of the head 6 is reversed by reversing the direction of the current passing through the voice coil 18.
Because the direction of the magnetic flux 22A induced along the first side 20A of the is voice coil 18 is opposite the direction of the magnetic flux 22B induced along the second side 20B, the first magnet 24A is magnetized from top to bottom with a magnetic polarity (N/S or S/N) that is opposite that of the second magnet 24B so that the magnetic fluxes 24A and 24B are aligned in the appropriate direction. In one embodiment, the first and second magnets (24A and 24B) are manufactured from separate pieces of magnetic material and then magnetized with the appropriate polarity N/S or S/N. In alternative embodiment, the first and second magnets (24A and 24B) are manufactured from a single piece of magnetic material and then magnetized with the appropriate polarity (N/S and S/N). Thus, the dashed line between the first and second magnets (24A and 24B) shown in FIG. 2 may represent a border between two separate pieces of magnet material, or a polarity border delineating two separate magnetized regions of a single piece of magnetic material.
The first and second magnets (24A and 24B) are housed within a rotary voice coil yoke 26, further details for which are illustrated in a perspective view in FIG. 2B and in a plane view in FIG. 2C. The yoke 26 comprises a top magnetic flux conductor 28A and a bottom magnetic flux conductor 28B. The first and second magnets (24A and 24B) are attached to an interior surface 30 of the top magnetic flux conductor 28A. The yoke 26 may further comprise a third magnet 32A and a forth magnet 32B attached to an interior surface 31 of the bottom magnetic flux conductor 28B. As shown in FIG. 2C, the top magnetic flux conductor 28A and the bottom magnetic flux conductor 28B form an air gap 34 between the magnets (24A, 24B, 32A and 32B). The polarity (N/S) of the magnets (24A, 24B, 32A and 32B) generates a multidirectional magnetic flux 36A and 36B with respect to the air gap 34. In the example shown in FIG. 2C, the direction of magnetic flux 36A is upward from magnet 32A to magnet 24A, and the direction of magnetic flux 36B is downward from magnet 24B to magnet 32B. The magnetic flux 36A interacts with the magnetic flux 22A of FIG. 2B generated by the first side 20A of the voice coil 18, and the magnetic flux 36B interacts with the magnetic flux 22B generated by the second side 20B of the voice coil 18.
There are drawbacks associated with the conventional rotary voice coil yoke design of FIGS. 2B and 2C. Namely, the magnets 24A, 24B, 32A and 32B represent a significant cost of the overall actuator assembly. In particular, the magnetic material itself is expensive and there is expense involved with magnetizing the magnetic material. In addition, the conventional two-piece yoke design increases the manufacturing cost of the disk drive due to the three step process required to manufacture the actuator assembly. First, the bottom magnetic flux conductor 28B is fastened to the base of the disk drive (e.g., glued or screwed down). Next, the actuator arm 10 is fastened onto the pivot 12 such that the voice coil 18 is positioned over the second and third magnet 32A and 32B. Finally, the top magnetic flux conductor 28A is fastened to the bottom magnetic flux conductor 28B (e.g., glued or screwed down) such that the first and second magnets 24A and 24B are positioned over the voice coil 18. This three step process increases the manufacturing time and therefore the manufacturing cost of the disk drive.
The cost of the rotary voice coil yoke design of FIGS. 2B and 2C can be reduced by eliminating the top magnets 24A and 24B or the bottom magnets 32A and 32B. However, the stray flux emanating from the top and bottom sides of the magnets interact with the top and bottom sides of the trapezoidal coil 18 shown in FIG. 2A which can excite resonances in the system leading to poor performance. Thus, the prior art typically employs top and bottom magnets so that the stray magnetic flux emanating from the top and bottom sides of the magnets is canceled.
It is also known to construct a voice coil motor by wrapping a voice coil around a middle conductor within a closed-ended yoke (low reluctance on both ends) comprising a top and bottom plate connected at the ends to form a closed housing for the middle conductor. This is illustrated in FIG. 3A which shows a top view of a closed-ended yoke 38 and a first and second voice coil 40A and 40B wrapped around a middle conductor 42. The first and second voice coils 40A and 40B are wrapped in opposite directions and magnets 44A and 44B are magnetized with opposite polarity. The construction of the closed-ended yoke 38 is similar to the yoke shown in FIG. 2A with the addition of a middle conductor 42 connected at both ends of the yoke within the housing. FIG. 3A also shows that two additional magnets 46A and 46B are attached to the back side of the closed-ended yoke 38 to generate flux which interacts with the back side of the voice coils 40A and 40B. A plane view of the closed-ended yoke 38 of FIG. 3A is shown in FIG. 3B. Only the first voice coil 40A is shown wrapped around the middle conductor 42. FIG. 3B also illustrates the bottom magnet 48A attached to the bottom plate of the closed-ended yoke 38.
With the closed-ended yoke structure of FIGS. 3A and 3B, guiding the magnetic flux through both ends of the yoke 38 increases the inductance of the voice coils 40A and 40B, thereby reducing performance of the voice coil motor by increasing the rise time of current through the voice coils 40A and 40B which in turn reduces the rise time of the magnetic flux induced by the voice coils 40A and 40B. More power is required to compensate for the increased inductance, which is less efficient.
There is, therefore, a need for a lower cost, more efficient voice coil motor for use in a disk drive.
The present invention may be regarded as a disk drive comprising a disk, an actuator arm comprising a head, and a voice coil motor for actuating the actuator arm to position the head radially over the disk. The voice coil motor comprises a first magnet for generating a first magnetic flux, and a rotary voice coil yoke comprising a magnetic flux conductor shaped to form an air gap with respect to the first magnet, the magnetic flux conductor comprising a first end having a first reluctance and a second end having a second reluctance, wherein the first reluctance is substantially lower than the second reluctance such that the magnetic flux conductor guides the first magnetic flux through the air gap and through the first end. A voice coil is wrapped around at least part of the magnetic flux conductor for conducting a current to generate a second magnetic flux such that at least part of the second magnetic flux is within the air gap for interacting with the first magnetic flux.
In one embodiment, the magnetic flux conductor comprises a substantially C shape It comprising a top prong and a bottom prong, and the voice coil is wrapped around the top prong of the C shape. In another embodiment, the voice coil is wrapped around the bottom prong. In yet another embodiment, the magnetic flux conductor comprises a substantially E shape comprising a top prong, a middle prong, and a bottom prong, and the voice coil is wrapped around the middle prong of the E shape. In still another embodiment, the voice coil motor comprises a second magnet for generating a second magnetic flux, the first magnet is attached to the top prong of the E shape, and the second magnet is attached to the bottom prong of the E shape.
The present invention may also be regarded as a rotary voice coil yoke for use in a voice coil motor. The rotary voice coil yoke comprising a magnetic flux conductor shaped to form an air gap with respect to a first magnet. The first magnet for generating a first magnetic flux, the magnetic flux conductor comprising a first end having a first reluctance and a second end having a second reluctance, wherein the first reluctance is substantially lower than the second reluctance such that the magnetic flux conductor guides the first magnetic flux through the air gap and through the first end.